Due to the rapidly growing market in portable electronics and hybrid vehicles, energy conversion and storage devices with high energy and power densities are in ever-increasing demand. Supercapacitors, also known as electrochemical capacitors, have attracted a great deal of attention due to their higher power densities, longer life cycles, and fast charge-discharge capabilities when compared to conventional energy storage devices such as Li-ion batteries. Unlike batteries, which are generally limited by slow reactions, supercapacitors store charges via highly reversible electric double-layer ion adsorption and/or fast faradic redox reactions, making them ideal energy storage candidates for portable electronics and hybrid vehicles where rapid energy capture and delivery are needed. Nevertheless, supercapacitors often suffer from lower energy densities when compared with Li-ion batteries.
Carbon nanomaterials (e.g., graphene and carbon nanotubes) have been explored as electrode materials for high-performance supercapacitors owing to their high specific surface area, high electrical conductivity, excellent chemical stability, and good environmental compatibility. In addition, introducing pseudocapacitive materials that are capable of fast and reversible redox reactions at the electrode surface, such as metal oxides and conductive polymers, into the electrodes has resulted in much higher capacitances compared to carbon-based materials alone.